The vote itself is a procedural vote to open up debate on a bill, not a vote on the bill itself. This means that even if this procedural vote passes, it is not guaranteed that a new healthcare bill will pass, though it makes it significantly more likely.

It is far from assured, however, that even this procedural vote will pass. To succeed, 50 yes votes are required. Democrats, who hold 48 votes, are unlikely to contribute any "yes" votes to the final count. Several Republicans have publicly voiced reservations about this procedural vote and as of now are considered undecided. Sen. Susan Collins (R-ME) is expected to be a no vote.

The Republicans can afford to lose 2 of their 52 members, but no more.

Update 3:32 pm: Voting has begun. Chants can still be heard in the hallway.

Update 2:40 pm: 50 votes have yet to be secured. Republican Senators Johnson (R-WI) and McCain (R-AZ) have yet to vote. The only GOP "no" votes so far have been Senators Collins (R-ME) and Murkowski (R-AK).

Update 3:01 pm: Sen. McCain has arrived and voted yes. Sen. Johnson followed immediately after with a yes vote. 50 votes have been secured, allowing debate to begin on the Senate bill.